In recent years, there has been a significant effort oriented towards reducing the size and weight of power supply systems. Size and weight are very important power-supply design goals, especially for applications in space systems such as satellite power systems.
Reducing the size and weight of power supply systems typically requires reducing the size and weight of magnetic elements such as inductors and transformers, which are normally the most voluminous power supply elements. Power supply systems which operate at higher switching frequencies generally require smaller magnetic elements, and thus one way to reduce power supply system size is to operate at as high a switching frequency as possible.
There is a trade-off for operating at higher switching frequencies, however. An increase in switching frequency results in increased power losses dissipated in the power switches as well as increased electromagnetic interference (EMI) noise and switching communication problems, due to the rapid ON/OFF switching of the semiconductor switches at high voltage and current levels.
These consequences of increased switching frequency are especially significant in conventional "hard-switched" power converters. Conventional pulse width-modulated (PWM) converters are "hard-switched" meaning that the main semiconductor switch is turned off and on while carrying substantial currents or blocking substantial voltage. The consequences of operating at higher switching frequencies put a practical upper bound on the usable frequency range for conventional hard-switched pulse-width-modulated (PWM) topologies.
A number of DC to DC switching converter topologies have been invented which attempt to increase switching frequency without increasing converter switching losses significantly. One method of reducing switching losses and EMI noise caused by high switching frequencies is the use of "resonant" switching techniques which overcome some of the switching losses of hard-switched PWM topologies. Resonant switching techniques comprise the inclusion of some type of LC subcircuit in series with a semiconductor switch which, when turned ON, creates a resonating subcircuit within the converter. Timing the ON/OFF control cycles of the resonant switch to correspond with particular voltage and current conditions across respective converter components during the switching cycle allows for operating the switch with zero-voltage across and zero current through the switch, which inherently reduces most of the switching losses attributed to hard-switched PWM converters. Resonant switching is also referred to as "soft" switching, "zero voltage" switching (ZVS).
Although resonant switching converters provide the advantage of reducing switching losses by switching with zero voltage across and zero current through the power switches, resonant converters also have some disadvantages over conventional hard-switching converters. These disadvantages include:
(1) Resonant converters require a more complicated control technique for timing the ON/OFF cycles of the primary and secondary power switches, and hence need more complicated control circuitry;
(2) Resonant converters require the addition of resonant magnetic circuit elements. In resonant circuits, typically a resonant inductor and capacitor are needed to form a resonant circuit circulating energy stored in the leakage inductance of the transformer.
(3) Resonant converters cause increased "circulating current" which is energy being circulated back and forth between the primary and secondary windings rather than being delivered out to the load. This circulating current results in higher conduction (I.sup.2 R) losses dissipated in circuit components in the primary and secondary.
(4) Resonant converters have a relatively large I.sup.2 R losses due to the shape of the secondary waveform. In a resonant converter, the secondary current rises sharply to a high peak current value, and then falls off sharply to zero. This waveform shape results in a relatively high RMS value of current and corresponding high I.sup.2 R losses in comparison to a current waveform with lower peak current transferring the same amount of energy. The resistive losses (I.sup.2 R) in the secondary winding and secondary circuit are proportional to the square of the RMS current. This effect is especially pronounced at light loads. For very light loads, the average value of secondary current I.sub.SEC is close to zero, but peak-to-peak value of I.sub.SEC will stay constant resulting in significant I.sup.2 R losses.
(5) The secondary current waveform generally includes a substantial output ripple current because there are substantial periods of time in which no secondary current is delivered to the output capacitor of the power converter. Specifically, during the drive cycles of a flyback power converter, no current flows through the secondary winding to the output capacitor. Consequently, there is zero power contribution from the power converter to the output capacitor during these time periods. During zero power contribution periods, the output capacitor will continue to provide the output current required by the load while receiving no current from the power transformer, which results in fluctuations in the output voltage of the output capacitor, which fluctuations are known as "output ripple voltage." This places stress on the output capacitor, puts undesirable ripple voltage on the output waveform, and dissipates power in the output capacitor. To reduce these negative consequences requires using a capacitor with very low equivalent series resistance (ESR) and very low equivalent series inductance (ESL).
U.S. Pat. No. 5,594,629 to Steigerwald discloses a circuit which helps to reduce some of these resonant converter disadvantages. This patent discloses a power converter which requires fewer components, a simpler control technique, and reduced circulating current by operating in a "natural" zero-voltage switching mode. In natural zero-voltage switching mode, the primary-side power switch is turned OFF before the secondary side drain current reverses, reducing the amount of circulating current. By timing the secondary side power switch to turn ON a short time after the primary side power switch has turned OFF, natural zero voltage switching is achieved.
U.S. Pat. No. 5,796,595 to Cross discloses an interleaved continuous flyback power converter system. The patent discloses using two interleaved power converters which reduces current ripple and I.sup.2 R losses. However, while this interleaved converter operates efficiently at high power levels, its efficiency is poor at lower power levels.
What is needed is a PWM converter which can be contained in a small volume and operate at high efficiency over a wide range of output power levels.